1. Field of Invention
The present invention relates to a flame-retardant wear-resistant (abrasion resistant) resin composition useful as an electrically insulating material, particularly for electric wires (cables) such as automobile electric wires. The resin composition of the invention does not generate harmful halogen-containing gas when burned and provides satisfactory characteristics such as flame resistance, wear-resistance, tensile strength, flexibility, pliability, etc., for electric wire insulation. The invention also relates to a method of preparing such a resin composition.
2. Description of Related Art
As insulating coating materials for electric wires (cables), polyvinyl chloride (PVC) is currently being used in general because it is easily obtainable and adaptable according to the purpose of use from heat-resistance to cold-resistance, and from hard to soft quality, by selecting the mixing materials, and can give the characteristics required for coating materials of excellent insulating performance, flame resistance, oil resistance, ozone resistance and water resistance.
However, because PVC is a polymer containing halogens, it generates harmful gas such as hydrogen chloride when burning, which may cause secondary damage such as metal corrosion and the like in an electric distribution system due to hydrogen chloride gas resulting from burning of the cable.
In addition, due to the recent increased recognition of environmental protection, reduction of industrial waste such as plastic materials, and recycling, even with PVC used for insulation material for automobile-use wire harness cables, hydrogen chloride gas resulting from incineration has become recognized as an important problem. Therefore, a non-halogen flame-retardant insulation material which does not generate harmful halogen gas on burning is currently sought.
At present, as non-halogen, flame-retarding material, a mixture in which a metallic hydroxide blended with a polyolefin resin is typical. But this material has inferior pliability and flexibility compared with PVC. Also, it requires blending with a large amount of metallic hydroxide which is the flame-resisting agent in order to obtain necessary flame retardance. However, as the amount of flame-resisting agent increases, mechanical strength such as wear-resistance and tensile strength is largely decreased, and problems of inferior coldproofing, flexibility at lower temperature and chalking resistance arise.
The decrease in wear-resistance is particularly significant for insulating materials of thin wall-thickness, e.g. 0.1-1 mm, used in automobile electric cables.
A flame-retarding, wear-resistant resin composition using metallic hydroxide as a flame-resisting agent and including a mixture of polyolefin of low crystallinity and thermoplastic elastomer as polymer components is proposed in JP-A-7-110912. This disclosure seeks to achieve flame-resistivity and tensile strength by obtaining heat resistance and flexibility with a thermoplastic elastomer blend, using a polyolefin of low crystallinity which gives good dispersibility of the metal hydroxides.
However, it is difficult to satisfy the flame resistance and wear-resistance requirements in a well balanced manner, due to larger amounts of flame-resisting agent blended for securing the flame resistance which consequently reduces wear-resistance.
In addition, JP-A-9-31271 discloses use of two ethylene-olefin (for example, propylene) copolymers which are different in melt flow rate, density and molecular weight distribution, in a blend with a flame-resisting agent. Dispersibility of the flame-resisting agent is obtained by use of low-density ethylene-olefin (for instance, propylene) copolymer, and wear-resistance by use of medium-density ethylene-olefin (for instance, propylene) copolymer. But, even with this resin composition, increasing the ratio of the low-density ethylene-olefin copolymer becomes necessary in order to blend sufficient flame-resisting agent, which also leads to a difficulty in balancing the flame-resistance against wear-resistance. Furthermore, with this resin composition, increasing the content of medium-density ethylene-olefin copolymer in order to increase wear-resistance leads to a decrease in elongation, thereby not providing suitable flexibility as an insulation material for electric wires.
There are other proposals in the art. U.S. Pat. No. 5,378,856 describes an insulating coating for a conductor wire compound comprised primarily of a high density polyethylene (100 parts), an ethylene-vinyl acetate polymer (20-100 parts), a low density polyethylene (0-60 parts) and a flame-retardant such as aluminium hydroxide. EP-A-370517 likewise describes a fire-retardant composition, useful as an insulating coating, comprising a thermoplastic resin, particularly an ethylene/ethyl acrylic copolymer (e.g., 100 parts), a low-density ethylene/1-hexene copolymer modified with a maleic anhydroxide (e.g., 30 parts) and magnesium hydroxide (e.g., 200 parts).